(1) Field of the Invention
The present invention relates to a color image processing apparatus and a color image processing method, for example, in which an input color signal composed of three components, namely CMY, is subjected to an UCR processing using a K-separation, for example.
(2) Description of Related Art
In general, an image forming apparatus such as a printer for outputting color images, characters or the like on a printing medium such as a printing sheet or the like, reproduces a particular color by appropriately blending three chromatic components (color signals) of CMY (Cyan, Magenta, and Yellow) or four chromatic components (color signals) of CMYK.
In such an image forming apparatus, it is desired to reduce the amount of color materials used in color printing as much as possible, since the color material is generally expensive. On the other hand, there is known a method of adjusting color reproduction in which a color image composed of the three chromatic components of CMY is added with a K (black)-separation (component), whereby the color image is adjusted in a desired manner. Further, there is a problem that when a color image is formed, a desired amount of color material cannot be fixed on a sheet of printing material due to the property of the medium or color material (ink, toner or the like).
If the three components of CMY are blended at an equal amount, theoretically, a color of gray or color near to gray can be produced. By using the nature of the color blending, any color can be subjected to a UCR (UCR) processing in which the gray component of three components of CMY is removed from the color and replaced with the K (black) component. With this processing, the color to be presented can be produced with a smaller amount of color materials of CMY components or the color image can be adjusted in terms of color reproduction.
FIG. 15 is a diagram conceptually illustrative of a general UCR processing. A general processing of UCR processing will hereinafter be described with reference to FIG. 15.
The initial step is to find the smallest content of component of the three CMY components which constitute an input color signal (in the example shown in FIG. 15, the smallest component is Y-component). Then, the predetermined rate (UCR rate) is multiplied with the smallest content to yield the amount used in K-separation, or the rate of UCR.
Then, the input color signal is subtracted by the amount of CMY components each equal to the UCR rate, and the input color signal is added with the K-separation component of which amount is equal to the UCR rate (K-separation adding amount). Thus, the UCR processing is carried out.
In the example shown in FIG. 15, the UCR processing is carried out at the UCR rate of 100%. If the UCR rate is set to 100%, as in the example shown in FIG. 15, a color signal of the amount equal to the smallest amount of color signal (in the example shown in FIG. 15, the smallest component is Y-component) of the respective CMY components constituting the input color signal is replaced with the K-separation as the UCR rate.
When the above-described UCR processing is carried out upon printing a color image, the amount of consumed color ink (CMY) can be saved and the color image can be formed economically.
If the UCR rate is set to 100% as in the above-described example, the black version is introduced over the whole concentration region. Therefore, it becomes difficult to take a good matching between the three color inks (CMY) and the black ink. Further, a light portion of the image is also added with a considerable amount of black component, the background texture of the image at the portion tends to be dirty due to the black component. For this reason, a skeleton black method in which the UCR rate is set to a relatively low level is ordinarily utilized.
However, according to the general method of the conventional UCR method, since a part of the CMY color components are replaced with the K-separation, the color reproducibility is greatly deteriorated. For example, the lightness of the color image can change from the separation before undergoing the UCR processing and the separation after undergoing the UCR processing. Also, there is also a problem that the deepness of the color image cannot be completely reproduced after the UCR processing.
Particularly, in a conventional UCR processing method, a parameter (UCR rate), arbitrarily determined depending on the input color signal, is not utilized but a predetermined parameter is utilized for the UCR processing. Therefore, it is difficult to improve the color reproducibility of an image after undergoing the UCR processing. That is, it is difficult to determine a parameter which can provide good balance for every color. Further, to determine a parameter for improving the color reproducibility depending on the input color signal requires try-and-error operation and an operator with highly developed skill.
Japanese Patent Laid-Open (kokai) No. HEI 7-87346 discloses an UCR processing method in which an inputted color signal is converted into a color signal on a color space which is independent of the characteristic of the apparatus and has a uniform property in terms of a visual sense, and an UCR rate is calculated by using the saturation signal of the converted color signal, whereby an accurate color reproduction is accomplished through a simple calculation without necessity of an adjustment in parameter based on the operator's experience.
In this method disclosed in Japanese Patent Laid-Open (kokai) No. HEI 7-87346, however, the color reproducibility is dependent of the color space with the uniform property in terms of a visual sense. Therefore, when a color is actually outputted from the printing device or the like, for example, if it is requested to produce a gradation from a monochrome tone to a primary color (i.e., a color represented by only one color component of CMY components) or to produce a gradation from the monochrome tone to a secondary color (i.e., a color represented by two color components of the CMY components), the UCR rate does not vary uniformly in response to the input color signal. Therefore, the color reproducibility sometimes loses smoothness.
Further, according to the above-described method, the parameter is determined depending on only the degree of saturation. Therefore, if color signals have different lightness or hue but have the same degree of saturation, the color signals will be subjected to the UCR processing with the same parameter, i.e., the same K-separation is added to the color signals. Thus, to a part of an image such as a skin of a human, where with a hard color reproducibility or muddy color representation is not desired, K-separation is also added uniformly, which tends to prevent the reproduction of a color image with a quality desired by the user.
Also, Japanese Patent Laid-Open (kokai) No. HEI 7-107307 discloses a method in which the degree of saturation is calculated based on the input color signal, and the degree of saturation is utilized as a parameter of the UCR processing, in order to improve the color reproducibility in a high saturation area. Also in the method of Japanese Patent Laid-Open (kokai) No. HEI 7-107307, however, the parameter is determined depending on only the degree of saturation. Therefore, since this method does not cope with the difference in lightness of hue of the image, it is difficult to create a color image with a quality desired by the user.